Read this U.S. Customs and Border Protection website:
"What documents, identification, and paperwork does a legal permanent resident (LPR) or Green Card holder need to travel internationally?
Lawful Permanent Residents (Green Card holders) do not need a passport to enter the United States, however, they may need a passport to enter another country. Please contact the embassy of the foreign country you will be traveling to for their requirements. Note: Canada does not require a passport from U.S. LPR's. "
